1. Field of the Invention
The present invention relates to metal surfaces comprising a thin glass- or ceramic-type protective layer having high chemical resistance and improved non-stick properties.
2. Discussion of Background Information
Vitreous or ceramic-type protective layers are usually applied to metallic moulded articles either by enamelling operations, by flame or plasma spray processes, printing processes such as silk screen printing, or by gas phase processes. In order to apply very thin layers practically exclusively gas-phase processes are suitable. In addition, it is also possible to apply so called sol-gel layers, for example by dip methods, the layer thicknesses of which are, however, limited to about 1 μm (see C. J. Brinker, G. W. Scherer: “Sol-Gel Science—The Physics and Chemistry of Sol-Gel-Processing”, Academic Press, Boston, San Diego, New York, Sydney (1990)). Due to the very small layer thickness said layers neither have sufficient mechanical nor chemical properties, such as with regard to the attack by acids or bases, corrosion resistance or dishwasher resistance. Gas phase depositions do usually not result in “pinhole”-free layers, because hermetically sealed layer structures can not be obtained due to the kinetically controlled crystal growth on the surface.
To achieve dense enamel layers glass compositions have to be used, which must melt easily at relatively low temperatures as compared to glass melting processes (usually above 1000° C.) to be capable to form a continuous coating on the metal surface. The melting temperatures are, however, usually still between 750 and 800° C., if toxicologically harmless enamels with a minimum requirement to sufficient chemical resistance, for example, to boiling water, weak acids or cooking foods, are to be obtained. Because of the high alkali content required for ease of meltability, such layers are generally of low chemical resistance and are only of limited durability, for example when cleaned in a dishwasher (so called “glass corrosion”). There is also the fact that because of the necessary large layer thickness, the expansion coefficients of said enamels have to be adjusted to a relatively large extent to the metallic moulded articles, thus severely impairing the selection of the composition, since otherwise tensions with crack formation and even chipping will be the result.
This is also one of the reasons why, for example, enamel on aluminium is rarely used in the household appliance sector, especially in cookware, since aluminium melts or deforms strongly at temperatures slightly above 600° C. Analogous relations are found for magnesium or magnesium aluminum alloys. The same also applies to metal elements which are composed of several components, if any of these components is originated from the light metals mentioned above.
It has been proven that the aforementioned disadvantages occur much less frequently when thin vitreous sol-gel coatings are applied, because they have different elastic properties due to their specific microstructure. Thus, for example, sol-gel layers with a thickness of up to 1 μm can readily be heated to a temperature of 500 to 700° C. without occurrence of cracking, e.g. on stainless steel.
Patent documents U.S. Pat. No. 6,162,498 and US-A1-20080118745 describe methods in which vitreous layers are described, which are relatively resistant to abrasion and oxidation corrosion, for instance, tarnishing of stainless steel. The methods comprise the preparation of a coating solution by hydrolysis and polycondensation of one or more silanes in the presence of colloidal silica sol and at least one component of the group of alkali and alkaline earth oxides and/or hydroxides; the application of the coating solution on a metal surface to form a layer, and a thermal densification to form a vitreous film.
According to U.S. Pat. No. 6,162,498 densification temperatures between 350 and 500° C. are used. US 20080118745 further describes the formation of deformable vitreous layers by densification of an alkali silicate containing layer in a two stage process at a preferred temperature of 500° C., the production of coating systems by dipping and spraying with layers in the range of from 5 to 10 μm and the application of the coatings on metal surfaces and metal elements, especially on stainless steel, but also on aluminium and aluminum alloys.
The coatings mentioned in the patent publications cited above have, however, only a limited service life against acids and even more so in an alkaline medium, there can be no talk of years of stability. This is, however, just what customers expect, guarantees of up to 15 years are not uncommon. These layers can already be removed quantitatively in a short time with dilute sodium hydroxide solution (15%) at moderate temperatures (60-90° C.) and have in no account a permanent dishwasher stability. This means that they are usually not suitable for the food sector or as protective coatings for applications at higher or lower pH values.
While it is known in the pertinent sol-gel literature that the chemical resistance can be significantly improved by incorporation of foreign ions acting catalytically and network compacting, such as for instance iron, titanium, aluminum and zirconium, however these elements have serious disadvantages with respect to the stability of the coating sols, because a sufficiently long “pot life” for a spray coating process can not be achieved due to the catalytic effect, and with respect to the thermal densification process itself.
In addition to the required mechanical, thermal and chemical permanent stability, a so-called “easy-to-clean” surface, i.e. a surface with non-stick properties, is also desired in many areas. In particular on objects which are used, for instance, for cooking, frying, roasting or grilling, this is an important issue.
Untreated metal surfaces have virtually no anti-adhesion properties, especially when heated. Food and oils adhere to and can then only be removed completely with considerable mechanical effort. According to the prior art this problem is reduced by the use of organic fluorine containing polymers (PTFE, “Teflon”). Surfaces coated in this manner exhibit excellent hydrophobic and oleophobic effects. However, it has long been known that surfaces coated with Teflon are not stable, neither mechanically nor thermally. Continuous use in kitchens has demonstrated that the “Teflon effect” decays very quickly, especially when using metallic scrapers, turners or spoons.
Moreover, PTFE is now classified as critical by food law, owing to the fact that high polymer PTFE rearranges at higher temperatures into volatile, ring-shaped, partially oxidized molecular structures (“ring-chain equilibrium”). These ingredients can then diffuse or migrate, e.g. into food or liquids which are in direct contact with the coated surface, or be inhaled directly. In particular, inhalation provably results in the impairment of health (“Teflon fever”, PTFE toxicosis). To eliminate this risk, the amount of PTFE in coatings should be significantly reduced or completely omitted.
It is an object of the invention to provide metallic substrates having a glass-, glass-ceramic- or ceramic-type coating with improved chemical resistance, in particular improved alkaline resistant and even permanent dishwasher resistant properties, which in addition should have a substantially better anti-adhesive property compared to an uncoated metal surface, although PTFE (“Teflon”) should be omitted completely.